I ran into an odd UNIX filename issue while writing Go code the other day.
Here’s a simplified example:
Let’s read a JSON file and unmarshal its contents into a struct in go. First, let’s set an environment variable with our file name to avoid hardcoded constants in our program.
export MY_FILE="/Users/dancorin/Desktop/test.json "
Now, let’s read the file into our struct:
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"os"
)
// Stuff struct holds the json contents
type Stuff struct {
Test string `json:"test"`
}
func main() {
stuff := Stuff{}
place := os.Getenv("MY_FILE")
data, err := ioutil.ReadFile(place)
if err != nil {
panic(err)
}
json.Unmarshal(data, &stuff)
fmt.Printf("%+v\n", stuff)
}
❯ go run program.go
panic: open /Users/dancorin/Desktop/test.json : no such file or directory
goroutine 1 [running]:
main.main()
/Users/dancorin/Desktop/program.go:20 +0x156
exit status 2
Looks like Go couldn’t find my file.
❯ pwd
/Users/dancorin/Desktop
❯ ls test*
test.json
The file definitely exists. What about its permissions?
❯ ls -ltrah test*
-rw-r--r-- 1 dancorin staff 18B May 9 15:56 test.json
Looks like the file is readable by my program too. So, what is happening?
❯ cat test.json
{"test": "stuff"}
I can see the file contents too.
❯ cat /Users/dancorin/Desktop/test.json
{"test": "stuff"}
I am using the proper path. Let’s check that Go is trying to read the correct file path.
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"os"
)
// Stuff struct holds the JSON contents
type Stuff struct {
Test string `json:"test"`
}
func main() {
stuff := Stuff{}
place := os.Getenv("MY_FILE")
fmt.Printf("PLACE: %s\n", place)
data, err := ioutil.ReadFile(place)
if err != nil {
panic(err)
}
json.Unmarshal(data, &stuff)
fmt.Printf("%+v\n", stuff)
}
Running the code:
❯ go run program.go
PLACE: /Users/dancorin/Desktop/test.json
panic: open /Users/dancorin/Desktop/test.json : no such file or directory
goroutine 1 [running]:
main.main()
/Users/dancorin/Desktop/program.go:21 +0x202
exit status 2
The value of the environment variable seems to be correct.
Let’s see if we can find any weird characters hiding in the string:
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"os"
)
// Stuff struct holds the JSON contents
type Stuff struct {
Test string `json:"test"`
}
func main() {
stuff := Stuff{}
place := os.Getenv("MY_FILE")
fmt.Printf(">%s<\n", place)
data, err := ioutil.ReadFile(place)
if err != nil {
panic(err)
}
json.Unmarshal(data, &stuff)
fmt.Printf("%+v\n", stuff)
}
❯ go run program.go
>/Users/dancorin/Desktop/test.json <
panic: open /Users/dancorin/Desktop/test.json : no such file or directory
goroutine 1 [running]:
main.main()
/Users/dancorin/Desktop/program.go:21 +0x202
exit status 2
It looks like there is an unexpected space showing up in >/Users/dancorin/Desktop/test.json <. Where is this coming from?
When we set our environment variable, it seems like we accidentally added a trailing space.
export MY_FILE="/Users/dancorin/Desktop/test.json "
Go is trying to tell us this:
panic: open /Users/dancorin/Desktop/test.json : no such file or directory
It’s just not that obvious that there is a space in there. Something like the following could have helped:
panic: open "/Users/dancorin/Desktop/test.json ": no such file or directory
UNIX makes this issue a little more confusing because it has no problem allowing you to create filenames with trailing spaces. We can resolve our issue by running
❯ cp test.json "test.json "
❯ go run program.go
>/Users/dancorin/Desktop/test.json <
{Test:stuff}
Or, better yet, we can fix our export command:
export MY_FILE="/Users/dancorin/Desktop/test.json"
I hope you never run into this one!